1. Field of the Invention
The invention relates to an accessory unit for a sewing machine which automatically sews pieces of ribbon material, in particular channelling strips, onto the sewing material in the correct position, wherein the sewing machine has a sewing head, a sewing needle which is guided therein and a presser foot, and wherein the accessory unit comprises a ribbon dispenser, a ribbon-guidance mechanism with a ribbon cutter along its length and a feed advance mechanism for the ribbon or severed pieces of ribbon.
2. Description of the Prior Art
An accessory unit of this type is known from DE-A-35 34 988. In this accessory unit the ribbon guidance mechanism and advance feed mechanism are formed jointly by an arrangement of a number of pairs of rollers of which at least one pair can be driven. The disadvantage of this accessory unit is that the ribbon guidance and advance feed mechanisms require on the one hand a relatively large amount of space for installation, and on the other are relatively complicated with respect to mechanical parts, whereby breakdowns can quite easily occur.
A further accessory unit for a sewing machine is known from DE-A-36 42 863.9 which is similarly designed for the application of ribbon materials. This accessory unit has a guide shaft with a small window on one side which can be slightly tilted around an axis horizontal to the sewing machine and which serves as a guidance mechanism. A motor-driven frictional wheel passes through this window into the inside of the shaft which either picks up the ribbon material and transports it further or is raised therefrom depending on the shaft's inclination. The unit displays a sewing or pressure foot independent of the guidance shaft which is a trough-shaped curved continuation of the shaft, with an intermediate space with no guidance, whose top surface is covered by an elastic tongue. The disadvantage of this accessory unit is that it too requires a large amount of installation space and is technically very complicated.
A further accessory unit of the type first mentioned is known from DE-A-37 02 747. In this accessory unit the ribbon conveyor is formed by a roller-driven continuous belt which essentially extends along a guide chute and the cutting device is arranged in the belt's direction of movement between the continuous belt and a second belt conveyor. The disadvantage of this accessory unit is that the belt conveyor requires a very high mechanical outlay and is subject to high wear and tear. Furthermore, during the operation of these types of belt conveyors there is slippage between the continuous belt and the ribbon to be conveyed which prevents an exact advance feed.
And finally, a further accessory unit for a sewing machine is known from DE-A-37 05 703 which is essentially formed by a vertically-moving shuttle with a ribbon take-up which is limited by gripping fingers. This accessory unit is designed especially for installation in the bed of a sewing machine or a work plate so as to feed a ribbon or section of ribbon to the underside of the sewing material. The installation of the accessory unit at the sewing head of a sewing machine is practically impossible since this unit has a very high horizontal working radius and requires a great deal of space for movement.